Two questions - 18F87J50

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- 21st March 2013, 15:10
Christopher4187

Two questions - 18F87J50

1. I'm using a 20 MHz crystal that has 4 pins, 2 ground and the 2 osc. Does it matter where I connect the two osc pins? In other words, does it matter if I connect osc1 where osc2 is supposed to go.......if that actually is supposed to happen?

2. The VDDcore pin is a little confusing.

I'm looking at the schematic for the 18F87J50 demo board (page 25) and it shows VDD being connected directly to VDDcore and then that point is connected in parallel with two capacitors, a 4.7uF and a .1uF.

Then I look at the 18F87J50 datasheet and on page 360 is describes how everything works. Is the only advantage that you don't have to connect all of the power pins as the power is supplied from the core? I also don't understand the advantage of using 2.5V as opposed to 3.3V. Perhaps for battery operated devices?

Anyhow, I don't want to use the core regulator. According to the datasheet, tying the ENVREG pin to ground will disable it. I got that. Then I can supply 3.3V to VDD but then the IO pins will run at 2.5V because it shows 2.5V being tied to VDDcore/Vcap?

It would seem that I do need to enable the core regulator but the datasheet only shows one cap with no value (I'm assuming it should be a ceramic .1uF) and the demo board sheet shows two caps.
- 21st March 2013, 17:24
spcw1234

Re: Two questions - 18F87J50

Quote:

Originally Posted by Christopher4187

1. I'm using a 20 MHz crystal that has 4 pins, 2 ground and the 2 osc. Does it matter where I connect the two osc pins? In other words, does it matter if I connect osc1 where osc2 is supposed to go.......if that actually is supposed to happen?

The osc can be put in either way. Not familiar with VDDcore.
- 25th March 2013, 10:46
Dave

Re: Two questions - 18F87J50

Chris, what voltage level is the supporting electronics going to run at? If it is 5 volts then I would enable the internal VCORE regulator and run the processor at 5 volts. If you want to conserve power and the support electonics can tolerate 3 volts to operate then by all means disable the internal regulator and provide your own 3 volt supply. I am currently developing a system that uses an 18F67K22 which has this exact feature. I have chosen to enable the internal regulator and drive all the supporting electronics at 5 volts. It works just fine.
- 25th March 2013, 15:11
rmteo

Re: Two questions - 18F87J50

Quote:

Originally Posted by Dave

Chris, what voltage level is the supporting electronics going to run at? If it is 5 volts then I would enable the internal VCORE regulator and run the processor at 5 volts.

Absolute maximum voltage for the PIC18F87J50 is 4.0V (recommended is 3.6v) - Section 28.0 of the data sheet.

Quote:

NOTICE: Stresses above those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. This is a stress rating only and functional operation of the device at those or any other conditions above those indicated in the operation listings of this specification is not implied. Exposure to maximum rating conditions for extended periods may affect device reliability.